1. Field of the Invention
The invention relates to the field of flexible structures and, in particular, to a design for an attachment fitting for such structures. The attachment fitting is particularly useful on pressurized structures such a lighter-than-air vehicles and inflatable boats.
2. Description of Related Art
On flexible structures, attachment fittings and the like are normally just sewn directly thereon much like a button on a shirt. Sometime the reinforcements are sewn or bonded on both sides to add support. However, such fittings stitched or bonded in such a manner tend to "rip" away from the structure when under high tension loads, again much like a button ripping off from a shirt. For example in U.S. Pat. No. 4,125,233, "Tethered Aerodynamic Balloon With Integral Fins" by J. A. Winker, et.al. an attachment fitting is disclosed in the form of a loop with end portions bonded to the outer surface of a balloon. The end portions are "criss-crossed" with reinforcing strips bonded thereover. U.S. Pat. No. 3,108,765, "Balloon Having an External Ballonet" by V. H. Stone discloses a somewhat similar design. In addition, the V. H. Stone attachment fitting incorporates a load distributing fitting in the end of loop with the sides of the loop reinforced and joined together. Because the fitting is subject to cyclic tension loads there is always the tendency to Peel away the reinforcements. Thus it is important to distribute such tension loads into the main structure. Additionally, if the flexible structure is pressurized, stitching the attachment fitting thereon will create an unacceptable leakage path.
Also of interest is U.S. Pat. No. 4,267,989, "Toy Hot Air Balloon" by K. D. Skaggs. K. D. Skaggs disclosed an attachment fitting that includes a hole in the wall of the structure perpendicular to the specific direction of the tension load. A first flexible member is included having a center portion folded in half forming a loop extending through the slot. Attached to the loop are end portions bonded to the inside surface of the wall of the structure. Second flexible members are bonded over these end portions of the first flexible member. This design better distributes the tension load from the loop to the flexible structure, via shear load through the bond between the end portions of the first flexible members. However, the hole in the structure is not sealed; thus it is unusable in a pressurized structure. As used in the toy balloon, it is attached to an un-pressurized cap at the top thereof. However, others have modified the K. D. Skaggs design by extending the second flexible member to completely cover the first flexible member so that the attachment fitting can be used on pressurized structures. However, these designs do not provide for optimizing the distribution of loads into the structure eliminating or reducing local stress concentrations. Furthermore, if the flexible structure is an inflatable boat or the gas bag of a lighter-than-air vehicle, failure of the joint should not cause a leak therein.
Thus it is a primary object of the subject invention to provide an attachment fitting for a flexible structure.
It is another primary object of the subject invention to provide an attachment fitting for a flexible structure wherein stress loads are distributed into the structure, eliminating local stress concentrations.
It is a further object of the subject invention to provide an attachment fitting for a pressurized flexible walled structure.